"Robotics in the field of surgery is no doubt taking us a step forward," says Jayender Jagadeesan, Graduate Research Assistant in the Department of Electrical Engineering at the University of Western Ontario, London, Canada. "With the cooperation of Contemporary Controls, my research group is studying the advantages in using the Mitsubishi PA-10 7C robot in minimal invasive surgery."
The Mitsubishi PA-10 is a general-purpose manipulator mounted to a mobile platform with seven degrees of freedom (DOFs), weighing only 40kg and having the ability to pick up items up to 10 kgs due to its high motor torques. A big part of this ambitious project was to create an ARCNET connection between the manipulator's controller and the standard personal computer which required an appropriate ARCNET card. Since the research group employed the Robot Control C Library (RCCL) under Windows® OS to control the manipulator, it was essential to use ARCNET to communicate between the PC and the controller of the PA10.
Jagadeesan and other research assistants believe ARCNET "is the winning situation" because ARCNET is a general-purpose technology for network communication. Research assistants have been able to leverage knowledge of ARCNET gained from other applications.
"This means open standards to communicate with robotics can be achieved and open programming languages like RCCL can be applied to program different types of robots," they said.
An ARCNET network interface module was not available from Mitsubishi. However, Contemporary Controls in Downers Grove, IL designed and manufactured these cards which ran under Windows OS as well as other operating systems. As Contemporary Controls' engineers understood it, the PA-10 was supplied with an ARCNET interface that used the COM20022 ARCNET controller chip, plus the HYC4000 line driver. The HYC4000 is an eight-pin device that converts the ARCNET signaling into an AC-coupled "back-plane mode" EIA-485 signal for transmission over twisted-pair cable at data rates from 10 Mbps down to 1.25 Mbps.
Mitsubishi offered an "Optical Conversion Board (OCB) designed to replace the HYC4000 line driver-thus, using fiber-optic cable as the network medium instead of twisted-pair. It was supplied with an 8-pin header as a direct pin-for-pin replacement for the HYC4000. Contemporary Controls' ARCNET PCI20U-4000 offered the HYC4000 transceiver as an option.
Jagadeesan said the initial challenge was to interface the OCB to the PCI20U. The control of the robot was accomplished by means of a single PCI20U module integrated in the robot's ARCNET architecture. "The transceiver on the PCI20U was replaced by the OCB with proper signal polarity and pin identity," he said. "The OCB was connected to the servo amplifiers through fiber-optic cables. This OCB was employed to reduce the amount of noise and to provide for fast data transfer rates. It converted the data packets to optical signals."
Contemporary Controls' universal voltage PCI20U card is compatible with both +3.3 Volt ("PCI-X") and older, 5-Volt computers. PCI-X is an enhancement to the original PCI Local Bus Specification, enabling devices to operate at speeds up to 133 MHz. The PCI bus allows for jumperless configuration and Plug and Play (PnP) operation. The module operates with either a NDIS driver or with a null stack driver in a Windows environment. This device exploits the new features of the COM20022. This includes 10 Mbps communications utilizing the various EIA-485 transceiver options as well as command chaining, sequential access to internal RAM, and duplicate Node ID detection.
In the ARCNET architecture, the host computer acts as the transmitting node and the servo amplifiers act as the receiving node.
The packets are sent in a particular format from the host to the servo. The servo receives the packet and decodes the torque value to each of the seven joints of the robot. There are seven CPUs which individually control each of the joints, acting as a node in the ARCNET network. "When a torque command is given to the servo driver from the computer, the data packet is decoded and the torque is provided to each of the seven joints for robot control," stated Jagadeesan. "The extra degree of freedom could be used to avoid obstacles and singular points where the control breaks down. It also has the ability of being mounted either from the ceiling or the walls, which provides additional freedom of movement." He explained that the robot has three categories of control: 1) Joint-based control-each of the seven joints is controlled to follow a desired trajectory 2) Cartesian control-the robot is controlled to follow a desired trajectory in Cartesian space and 3) Force Control-the robot is controlled based on the forces, which are felt at the tip as measured by the 6-DOF force sensor.
Jagadeesan concluded that there are significant advantages in employing robots like the PA-10 for minimal invasive surgery. "Robots help in reducing the operating time, thereby reducing costs," he said. " The amount of trauma involved is far less compared to conventional surgeries since the incision size is very small, leading to quicker discharge of the patient. For extremely complicated surgeries (like operating on babies or performing brain surgery), it is essential to keep the hand as still as possible without any tremors. We know that it is impossible to keep the human hand steady continually. However, the robot could be programmed to cancel out these tremors which can be considered high frequency components."
The Mitsubishi PA-10 is programmed according to how a surgeon would function in the same condition. "In fact, most robots being developed in the medical field are the master-slave type: the surgeon works on the master, while the robot follows the motion of the surgeon's fingers," said Jagadeesan. "Surgeons and robots like the PA-10 will continue to work together, leading to more sophisticated methods in medical applications. This in turn means that networking components like the PCI20U will continue to help with these evolving requirements."